As described in Patent Document 1, for example, a combustion pattern by compression ignition of an air-fuel mixture in a cylinder has been known as a technology for improving both of exhaust emission performance and thermal efficiency of an engine. However, the compression ignition combustion involves a drastic pressure increase with an increase in load of the engine, and leads to increase in combustion noise. Thus, as described in Patent Document 1, the compression ignition combustion engine generally employs, in place of the compression ignition, spark ignition by driving a spark plug in a high-load operation range.
Patent Document 2 describes an engine which performs the compression ignition combustion in a low-load, low-speed range like the engine of Patent Document 1. This engine keeps a high temperature burnt gas in the cylinder by adjusting a period in which an intake valve and an exhaust valve are open in a range of compression ignition combustion to increase the temperature in the cylinder and accelerate compression auto-ignition combustion. In a high-load, high-speed range in the range of compression ignition combustion, the engine advances the timing of opening the intake valve to blow the burnt gas in the cylinder back to an intake port, and then reintroduces the burnt gas into the cylinder together with fresh air. Thus, the fresh air reduces the temperature of the burnt gas, and a compression end temperature relatively decreases in the high-load, high-speed range, thereby reducing sudden pressure increase by the compression ignition combustion.